Introduction
Radiation is a broad term that covers nearly every form of energy traveling through another medium.
Sound and the waves of the seas are radiation too.
When people speak of "radiation" they usually mean "electromagnetic radiation", or actually its subcategory "ionizing radiation", which is what this article is about.
Let's first take a brief look at electromagnetic radiation in general.
Like other forms of radiation, it consists of traveling energy, in this case with electrical and magnetic properties.
It consists of electrically charged subatomic particles like electrons and protons.
These particles tend to travel at high speed.
In a vacuum, they reach their maximum speed: light speed, 300,000 kilometers per second.
But it is not just particles streaming about; some of its behavior is better described by looking at it as waves of energy instead.
Physicists use both the particles view and the wave view and switch between them freely, which is what I will do also.
Let's start with the wave-view.
Wavelength, frequency and energy
Radiation as waves has the same properties as sound-, water- and other waves do: a wavelength and opposed to that, a frequency. In fact, radiation comes in a whole spectrum of them. For electromagnetic radiation, physicists historically divide that spectrum into segments, which are listed below.
| Name | Wavelength | Frequency |
| Gamma rays | < 10 pm | > 30 EHz |
| X-rays | 10 pm - 10 nm | 30 PHz - 30 EHz |
| Ultraviolet light | 10 - 400 nm | 790 THz - 30 PHz |
| Visible light | 380 - 760 nm | 400 - 790 THz |
| Infared | 760 nm - 1 mm | 300 GHz - 400 THz |
| Microwaves | 1 m - 1 m | 300 MHz - 300 GHz |
| Radiowaves | 1 m - 100,000 km | 30 Hertz - 300 GHz |
The list above contains many familiar terms.
At the lower end of the spectrum are types of radiation that we don't consider harmful at all.
In fact many, like radio waves, surge around and through us every day, evoking no anxiety except with the paranoid.
That relaxed feeling is justified because they are not harmful at all to humans, animals or plants.
They are forms of "non-ionizing" radiation, too weak in energy to cause significant damage to organisms.
The higher end of the spectrum, including the gamma rays, x-rays and the upper part of ultraviolet radiation, are "ionizing".
These types of radiation carry enough energy to knock electrons off atoms and cause serious damage to materials, including living tissue.
Ionizing radiation is produced only at extreme conditions.
It is a product of the nuclear fusion process in stars, like our sun, but also of nuclear fission like in our nuclear reactors and bombs.
Radiation by x-ray scans, nuclear waste and explosions is artificial, but there are natural forms of radiation too.
There are radioactive materials in the Earth itself, slowly decaying and emitting radiation.
And of course we are daily bombarded by the rays of the sun, which include a significant part of ionizing radiation.
The atmosphere partially protects us from the latter, but not completely.
So if we are continually exposed to harmful radiation, why don't we develop radiation disease?
That is because the level of radiation that we normally suffer is quite low and seldom causes problems.
But what level is safe and what is not?
Time to talk numbers.
Units
Radioactivity
The SI (Internal System of units) unit of radioactivity is the becquerel (Bq).
One becquerel stands for one particle of radiation per second.
The actual number of Bq emitted by a piece of radioactive material depends on its mass and the type of material.
For example, one kilogram of potassium contains 11.8 grams of the radioactive potassium-40 and emits 3.2e6 = 3.2 MBq.
Historically, other units have been used and are still used today.
I list them here so that you can convert numbers:
1 Curie (Ci) = 3.7e10 Bq = 37 GBq; 1 Rutherford (rd) = 10e6 Bq = 1 MBq.
Radiation dose
Radioactivity tells only part of the story.
What is important is the amount of radiation that a person receives, the radiation dose.
There are two SI units for measuring this.
The first is the gray (Gy), equivalent to 1 Joule of energy absorbed by 1 kg of body mass.
The second is the Sievert (Sv), which confusingly is also 1 J / kg.
The difference is that the Gy represents the raw radiation dose, while the Sv is corrected for biological effects.
For x-rays and gamma rays 1 Sv = 1 Gy, but for protons 1 Sv = 0.5 Gy and for alpha particles 1 Sv = 0.05 Gy,
as these are less energetic than beta and gamma particles and cause less damage to the body.
Here too there are historical units lingering around:
1 rad = 0.01 Gy; 1 rem = 0.01 Sv.
Doctors discern "absorbed dose", "committed / equivalent dose" and "effective dose" and point out that some body tissue is more vulnerable to radiation damage than others,
but that goes beyond the scope of this article.
I will stick to "absorbed dose" to keep things simple.
Dangers
Long term exposure to ionizing radiation increases the risk of several types of cancer. Short term but heavy exposure can wreck the cells in your body so much that you may become acutely ill. Both categories are lumped together under the term "radiation disease". This is essentially a risk and therefore cannot be fully quantified. It is not possible to say "radiation below this treshhold is safe and above is unsafe". What can be said are things like "this radiation dose increases the risk of cancer by x% over the lifetime of an average person". And even those numbers are not very hard. The reason of course is that ionizing radiation is so dangerous that scientists cannot experiment much with it, so there is little opportunity to gain knowledge. However, some decades of nuclear energy, several tests by ruthless governments and a few nuclear accidents have provided us with substantial insight.
Normal radiation
The table below lists the radiation dose that average person gathers, measured in mSv per year. Numbers tend to vary a lot depending on where you live, what you gather professionally and how many heavy medical scans you endure.
| Source | Normal range | Global average | Remarks |
| Cosmic radiation | 0.3 - 1.0 | 0.3900 | Least at sea level, most at high altitude |
| Air | 0.2 - 10.0 | 1.2600 | Mainly from indoor accumulation of radon gas |
| Terrestrial | 0.3 - 1.0 | 0.4800 | Depends on local radioactivity of the soil and building materials of houses |
| Body | 0.2 - 1.0 | 0.2900 | Mostly from radioactive materials in food (potassium, carbon) |
| Medical | 0.03 - 2.0 | 0.6000 | Mainly caused by CT-scans |
| Nuclear fallout | 0 - 1 | 0.0070 | Highest during the time of above ground nuclear tests |
| Other man-made | 0 - 20 | 0.0052 | Suffered by professionals in nuclear plants, some miners, airline pilots |
| TOTAL | 3 |
It is not certain exactly how dangerous these 'normal' levels are. We know that you need not worry about them much; most forms of cancer have a biological / chemical origin, not a radiation one. The USA has set a safety limit of 20 mSv / year for people who professionally work with radiation, for instance in nuclear plants. For the general public, the limit is much lower, at 1 mSv / year. The latter number is little more than the total of man-made radiation listed in the table above.
Abnormal radiation
Sadly, better data is available for acute high levels of radiation, mainly gathered from the victims of the two nuclear bombs dropped on Hiroshima and Nagasaki in 1945.
It is estimated that people at 1 kilometer distance of the explosions suffered a dose of 4 Gy, with corresponding higher and lower amounts closer by and further away.
From this, doctors derived that an acute dose of less than 0.25 Gy has no observable effects;
a dose up to 1 Gy causes temporary changes in the blood but no illness;
1 - 2 Gy causes sickness in most cases, fatal for a few percent;
2 - 6 Gy will make most people sick and kill a large proportion;
higher doses are almost always fatal within weeks or even days.
It is not just nuclear bombs that are an acute danger.
Accidents with nuclear reactors are not as explosive, but reach high levels of radioactivity too.
The nuclear accident at Chernobyl in 1986 for example, caused radiation levels of 300 Sv / hour in the vicinity of the reactor core,
lowering to 10 in the nearby area and about 0.04 in the control room.
During the Fukushima accident in 2011 radiation levels were as high as 1 Sv / hour in the immediate vicinity of the damaged reactors.
Radioactive decay
The immediately radiation resulting from nuclear explosions or leaks is a threat to the health of the people who work to contain the pollution from spilling out.
Radiation, like all energy, decreases in intensity in proportion to the square of the distance to its source, so at long distances its immediate effects are negligible.
But there is a second, slower and more wide-spreading danger: the infamous 'fallout'.
Both nuclear explosions and accidents spill radioactive materials into the surroundings.
These consists of radioactive isotopes that gradually, over time, suffer from radioactive decay and send out radiation.
Because they are chemically no different than stable isotopes, they spread through the environment.
They are carried through the air by the wind, spill down into rivers and oceans.
Some become part of the food chain and are accumulated in increasing quantities by the members of that food chain.
So humans, more or less at the end of that, gather the most of these radioactive materials.
As said, this matter does not radiate out all its energy at once, but over time, steadily diminishing in intensity.
That effect is inherent to the way that radioactive decay works.
Almost all material elements have several isotopes of different atomic weight.
Some of these are unstable: sooner or later they partially disintegrate, decaying into a lighter element and sending out the remaining matter/energy as radiation.
It is impossible to predict when such an atom will decay, but over large number things tend to average out
and even a small piece of matter contains so many atoms that its radiation intensity is quite predictable.
It is not constant, however.
Physicists use the term 'half-life' to denote the period in which half of the atoms of a given piece of radioactive material will decay.
After this, in the second half-life period, half of the remaining half, or a quarter of the total, will decay.
In the third half-life period an eighth, then a sixteenth and so on.
This means that the radiation emitted by radioactive material steadily decreases.
The original intensity and the half-life depend on the material.
Iodine-131 for instance has a half-life of just 8 days, initially shining intensely but quickly diminishing.
Uranium-238 on the other hand, which is a natural material, has a half-life of 4.5 billion years, decaying only very slowly.
Protection
Ionizing radiation is feared because it cannot be sensed with the human senses, only measured by artificial detectors like the geigerteller.
Most people do not have access to those, so how do you protect yourself?
The best way is to use common sense.
If a nuclear accident happens near to you, get a sizeable distance between yourself and the site of the trouble.
Move out of the way of wind and streams of water that carry radioactive material.
If you are already at a reasonable distance, stay indoors for a while.
Do not eat food from the area around the accident.
You may also want to take preventive medicine like iodine tablets.
The iodine in the tablets saturates the body, causing the body to take in less iodine from food, which may have been contaminated.
But your best friends are space and time.
The further away you are, the larger the area where acute radiation and radioactive materials must spread over and the more their concentration will diminish.
And because of the half-life, radioactivity also diminishes over time.
For example the resulting fallout of the Chernobyl disaster peaked at 5 μSv / hour in Finland, about the same as the cosmic background radiation received by an airplane flying at 12 kilometers altitude.
Still a risky number, but 60,000 times less than the radiation levels right next to the reactor itself.
At the time of writing, 27 years after the accident, radiation levels outside the Ukraine are more or less back to normal.